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The Raw Compressor

The Raw Compressor can reduce image storage requirements by converting an image from one pixel depth to another. For example, converting a 32-bit image to 16-bit format achieves a 2:1 compression ratio. The Raw Compressor can also convert a 32-bit image to 24-bit format by dropping the pad byte. This achieves a 4:3 compression with no loss of quality. The Raw Compressor accomplishes this conversion quickly, and the resulting image retains excellent image quality in most cases.

The Image Compression Manager often uses the Raw Compressor to extend the capabilities of other compressors. For example, the Photo Compressor works directly with only 32-bit color images and 8-bit grayscale images. For color images, the Image Compression Manager uses the Raw Compressor to convert the pixel depth of the original image to 32-bit color or to convert the 32-bit decompressed image to another pixel depth for display.

Image quality can deteriorate when the pixel depth is reduced; however, this technique is generally lossless when converting from a lower pixel depth to a higher depth. With 1, 2, 4, 8, and 24-bit images, the Raw Compressor allows colors to be mapped through a color table.

Note that the resulting image may be larger than the corresponding pixel image in PICT format, because QuickDraw stores PICT images in a run-length encoded format.

Performance figures for the Raw Compressor are dependent upon the source and destination pixel depths. (The Raw Compressor is signified by the None option in the standard compression dialog box.)

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